Collins, a member of the Senate Intelligence Committee, also said that Mueller "must be allowed to complete his work without interference" regardless of who is the attorney general.

It is imperative that the Administration not impede the Mueller investigation. I’m concerned Rod Rosenstein will no longer be overseeing the probe. Special Counsel Mueller must be allowed to complete his work without interference—regardless of who is AG.

"I want to thank Jeff Sessions for his service to our country as Attorney General. Under Acting Attorney General Matthew Whitaker, it is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded," he said in a tweet.

I want to thank Jeff Sessions for his service to our country as Attorney General. Under Acting Attorney General Matthew Whitaker, it is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded.

Romney and Collins's pushback on Wednesday comes after Trump announced that he was ousting Sessions from the top Justice Department spot and that Whitaker, his chief of staff, will take over in an acting role including overseeing the Russia probe.

Whitaker's ascendence immediately set off alarm bells among congressional Democrats. He's previously criticized the Mueller investigation, including warning in a 2017 op-ed that Mueller was "dangerously close to crossing" a line if he looked into the Trump family's finances.